I work in email and someone in an online community mentioned that they couldn't find a web-based DKIM verification utility that supported either uploading an email or POSTing one via a form. It's a 15 minute script, so I decided to put one up in case it'd also be useful for anyone else. Please forgive my lack of care for making this look pretty.
N.B. if an uploaded file is sent, it is used instead of the contents of the textarea. Use one or the other, not both. I don't do any serious error handling, so if you get nothing back, you probably didn't fill something out right. I store the uploaded file temporarily, but all files are explicitly unlinked when they're finished validating.